1. Field of the Invention
The present invention relates to an imaging apparatus utilizing the phase of an X-ray.
2. Description of the Related Art
X-rays, which are highly transmissive through a substance, are widely utilized in medical and industrial fields in image pickup units that capture an image of the inner structure of an object to be inspected. Nowadays, an imaging method, in which the phase shift of an X-ray occurring due to transmission of the X-ray through the object is utilized (X-ray phase imaging), has been studied and developed.
As one of the technologies in which the phase shift of the X-ray is utilized, an imaging technology in which a periodic pattern is formed by utilizing a grating has been proposed. As examples of such a technology, the following methods are known: a method in which the shadow of the grating is projected onto the detector described in International Publication No. WO/1995/005725, and a Talbot interference method described in U.S. Pat. No. 5,812,629.
In the example described in International Publication No. WO/1995/005725, an X-ray is spatially split into a number of beams using a grating, an object to be inspected is irradiated with the split beams, and a periodic pattern formed by the X-ray beams having been transmitted through the object are detected by a detector. The X-ray beams are refracted in accordance with a phase change due to transmission through the object. The refraction causes the positions of the X-ray beams to shift. This positional shift of the X-ray beams can be detected with the detector as a change in the periodic pattern caused by the object.
In the above-described method, the spatial period of the X-ray beams split by the grating is an important parameter that determines the performance and the size of an X-ray imaging apparatus. In general, a comparatively small period of X-ray beams, for example, several tens of μm, is desirable with respect to detection sensitivity for refraction of X-ray beams, spatial resolution, capability of separating information about refraction from information about absorption, the size of the apparatus, and the like.
However, since the spatial resolution of a detector used for X-ray imaging apparatuses is typically several tens of μm or equal to or larger than 100 μm, the period of the X-ray beams needs to be designed in accordance with the spatial resolution of the detector.
In the example described in U.S. Pat. No. 5,812,629, a grating uses a diffraction grating and the phase shift of an X-ray caused by an object to be detected is detected by forming an interference pattern called a self image by utilizing the Talbot effect and detecting a change in the interference pattern formed due to transmission of X-ray beams through the object. In the Talbot interference method, an interference pattern having a several μm period is typically formed on an X-ray receiving surface of a detector separated from the grating by several hundred mm. The interference pattern is one of periodic patterns formed due to interference of the X-ray (light). Such an imaging method is referred to as the Talbot interference method in the field of X-ray phase imaging.
However, the spatial resolution of a detector used for X-ray imaging apparatuses is typically several tens of μm or equal to or larger than 100 μm. Thus, it is difficult to directly detect an interference pattern formed by utilizing a general Talbot interference method.
In order to address the above-described problem, in the technology described in U.S. Pat. No. 5,812,629, a shield grating is provided at a position where the interference pattern is formed. This shield grating has a periodic structure, the period of which is substantially equal to that of the interference pattern. This allows a local change in the interference pattern to be indirectly detected.
However, use of the shield grating causes the apparatus to have a complex structure.
For these reasons, in an X-ray imaging apparatus for imaging the object by using the periodic pattern, it is desired that a periodic pattern with a smaller period can be directly detected by the detector.